MBTA. Groundwater Action Plan. Dr. Steve J. Poulos, P.E.

Transcription

1 MBTA Groundwater Action Plan Dr. Steve J. Poulos, P.E.

2 Topics to Cover Groundwater System in Back Bay Description of MBTA s Interim Step Steps to select MBTA s Long Term Solution

3 Groundwater System in Back Bay

4 Groundwater in Back Bay/South End Fill The groundwater in the Back Bay fill is, for the most part, contained within certain lowpermeability boundaries that form the Back Bay. Addition or removal of groundwater in one location will affect wide areas within the fill to some degree.

5 1858 Photograph of Back Bay (from State House) From Gaining Ground (2003), Figures 7.19 and 7.20

6 2001 Street Map with 1630 Shoreline of Back Bay From Gaining Ground (2003), Figure 7.1

7 1992 Bird s Eye View with 1630 Shoreline of Back Bay Study Area From Gaining Ground (2003), Figure 7.9 (Courtesy of Kane Maps)

8 Groundwater in Back Bay/South End Fill The infiltration of groundwater into the fill, if it were not covered with impervious or other low permeability surfaces, would average approximately 16 in./yr. This infiltration is equivalent to continuous inflow from rainfall of 500 gpm, on average, within the 600 acres bounded by Beacon Street (Mill Dam), Arlington Street, Washington Street and Massachusetts Avenue.

9 Groundwater in Back Bay/South End Fill If 500 gpm of infiltration were to occur with no removal, the groundwater level would be at the surface of the fill. Due to present surface cover, we estimate the infiltration to be 3-4 in./yr, on average, and that about 100 gpm to 125 gpm now enters the fill. As a result of the surface cover, we have lost about 375 gpm of infiltration into the groundwater.

10 Groundwater in Back Bay/South End Fill Current sources of inflow to groundwater: Infiltration from rainfall Leaky water mains Leaky sewer pipes above El. 5 BCB Leakage from Charles River Existing recharge systems

11 Groundwater in Back Bay/South End Fill Current causes of groundwater removal: Construction dewatering Leakage into deep sewer pipes Sump pumping from residences Sump pumping from deep basements Pumping for transportation corridors Flow into lower sand layer Loss of infiltration due to surface cover

12 Groundwater in Back Bay/South End Fill But the groundwater levels are below El. 5 BCB in some zones, and nearby untreated timber piles are exposed to decay. At each zone of low groundwater levels the balance between inflow and outflow is changed due to local groundwater removal. At each location of low groundwater the causes, and therefore the methods used to solve the problem, both vary.

13 Groundwater System in Back Bay/South End Questions?

14 Description of MBTA s Interim Step

15 Purposes of Interim Step To make an in-situ test of an interim recharge system using two new wells while continuing to recharge the two existing wells on St. Charles St. To raise groundwater levels nearby. To make measurements needed to design and estimate cost of a permanent groundwater mitigation system. To help identify nearby significant causes of groundwater lowering.

16 View of Interim Recharge System near Back Bay Station

17 Details of Testing 1 of 3 Monitor selected existing observation wells weekly prior to starting new recharge. Install two new recharge wells, one at the north end of Cazenove St. and one on west side of Berkeley St. toward the tracks. Install one new observation well. Survey owners nearby to learn basement elevations and presence of sump pumps.

18 Survey of Owners Completed surveys will be collected tonight. Extra copies of the survey are at door. Staff are available to help you complete the surveys after this presentation.

19 Closer View of Interim Recharge System

20 Details of Testing 2 of 3 Recharge well on Cazenove St. at ~5 gpm using City water and monitor effect on groundwater levels. Estimate how much the flow rate can be increased without affecting low basements. Increase recharge rate accordingly. Measure effect of increased recharge rate on groundwater levels. Follow the same steps for the well on Berkeley St.

21

22

23

24

25

26

27 Details of Testing 3 of 3 After testing is complete, for the interim step, set the flow rates appropriately for continued recharging in the two new and two existing recharge wells. Continue groundwater level measurements and make adjustments as needed to keep the wells operational until a permanent solution is installed and operating perhaps in three years.

28 Schedule Interim Step July to August 2006 August 2006 July to September 2006 November 2006 September 2006 to December 2006 December 2006 to December 2009 Permitting for Monitoring Well Install Monitoring Well Permitting for Recharge Wells Install Recharge Wells Weekly Groundwater Measurements Monthly Groundwater Measurements

29 Solutions that could be Implemented now by Others Reduce pumping as much as practicable. Direct roof drains into recharge or dry wells. Repair leaks in deep sewers. Repair specific leaks if practicable.

30 Description of MBTA s Interim Step Questions?

31 Steps to Select MBTA s Long Term Solution

32 Some Long Term Solutions for the MBTA Reduce pumping as much as practicable. Recharge into the fill any groundwater that is pumped to keep the tracks dry. Repair specific leaks if practicable. Install additional permanent recharge if needed. Install impervious walls below ground if needed.

33 Steps to be taken by MBTA to Select a Long Term Solution Preliminary evaluation of each option to eliminate those that are not feasible. Obtain necessary field data and carry out design analyses to evaluate remaining options. For the remaining options, prepare preliminary designs so that costs of construction and maintenance can be satisfactorily estimated. Select not more than three options for detailed evaluation. Select recommended option based on evaluation of advantages and disadvantages of each.

34 Steps being taken by MBTA to Select a Long Term Solution Review MBTA plans to understand where leakage may occur. Measure water flow in manholes within track drainage system. Measure water quality in manholes and at selected pumping stations. Use results of Interim Step recharging to obtain good estimate of permeability of fill and check against direct insitu measurement in the new observation well. Use results of Interim Step to help find any local causes of groundwater removal and the estimated rates of removal.

35 Steps being taken by MBTA to Select a Long Term Solution Evaluate solutions applicable to mitigate or eliminate effect of groundwater pumping. Select a solution that is effective, is environmentally appropriate, and has acceptable initial and long-term costs.

36 Draft Schedule Long Term Solution Fall 2006 Winter 2006 Winter 2006 Spring 2007 Spring 2007 Winter 2007 Winter 2007 Winter 2008 Winter 2008 Spring 2009 Spring 2009 Winter 2009 Evaluate, Recommend, Public Involvement Plan, Design, Public Involvement Bid, Permit, Public Involvement Construct, Public Involvement Observe, Monitor, Public Involvement Phase Out Interim Step, Public Involvement

37 Draft Schedule Long Term Solution Recommendation August 2006 December 2006 January 2007 February 2007 March 2007 Data Acquisition and Public Involvement Data Evaluation and Analysis, and Public Involvement Recommend Long Term Solution and Public Involvement

38 Steps to Select MBTA s Long Term Solution Questions?